The invention relates to a device for steering a vehicle, especially a road vehicle, and for maintaining a controlled course, by means of a feedback type steering control mechanism.
In known experimental road vehicles of this kind, the position of the vehicle in relation to the road is detected (e.g., with video cameras) and on the basis of these data the steering of the vehicle on the road is controlled so that it maintains a specific course. Detection of the vehicle's position in relation to the underlying surface can be achieved by image analysis methods which determine the position of the vehicle in relation to the roadway from a sequence of video images. An image analysis method of this kind is described in U. Franke, Real time 3D-road modeling for autonomous vehicle guidance, Theory & Applications of Image Analysis, Hrsg. P. Johansen and S. Olsen, World Scientific, 1992.
Known steering control methods in experimental vehicles can be divided into two groups. In a first group the driver can switch between manual and automatic steering modes, the manual mode requiring normal steering wheel actuation by the driver, while in automatic mode (e.g., by means of a device of the generic type) the steering angle or the steering angle speed alone is prescribed by a steering control device without the driver's having a direct means of intervention. In the control method of the second group, a controller specifies a steering torque which is added to that applied by the driver; this arrangement is intended to give the driver information on the correct course, or even to realize largely automatic driving when the steering wheel is released.
A steering device of the latter kind is disclosed in German Patent Document DE 34 14 891 C2, in which switching over can occur between manual steering and automatic transverse guidance. The device for automatic transverse guidance is coupled to a hydraulic circuit of the driver actuated power steering system, so that in an emergency the driver can override the automatic transverse guidance. To do so, the driver must apply to the steering wheel a steering torque which is greater than that produced by the automatic transverse guidance control. In this process, automatic transverse guidance produces in each case a steering force value which corresponds completely to that value which is produced from the detected control deviation of the position of the vehicle.
One object of the present invention is to provide a device for steering and maintaining the course of a vehicle in which, on the one hand the driver retains his or her ability to intervene in the steering of the vehicle, (particularly for relatively large steering maneuvers), and which on the other hand relieves the driver largely of continuous steering intervention.
This problem is solved by the steering control according to the invention in which the steering angle setting device combines the steering setting requested by the control and the steering setting requested by the driver, with limitation of the steering angle adjustment requested by the control device. In this manner, it is possible for the driver on the one hand to retain control over relatively large changes in the vehicle steering (e.g., in order to carry out avoidance maneuvers or to change lanes), while on the other hand he or she is relieved of normally constant necessity of making small steering corrections on approximately straight or slightly curved sections of road. With this device, small steering movements to maintain a prescribed set position of the vehicle are carried out by the control device in a course-maintaining fashion, whilst the driver simultaneously retains the ability to perform relatively large steering movements which are combined with the steering behavior induced by the control. At the same time, a very uniform transition between purely manual control and purely automatic control is also achieved.
In a preferred embodiment of the invention, a distance which is to be maintained from a lane boundary, (such as a center strip of a roadway) serves as a set value for the position of the vehicle; and the instantaneous distance, the angle between the longitudinal axis of the vehicle and the tangent to the lane boundary and the curvature of the lane boundary are determined in each case in order to maintain the set value.
In another advantageous embodiment of the invention, the set distance to position the vehicle (i.e., distances from the lane boundary, set in particular by the driver in a preceding period) is adjusted so that during a journey, different set distances can be selected in accordance with the preceding driving behavior. According to a dynamic adjustment feature of the invention, the control is adapted to transitions between non-steady-state sections of a journey in a gliding fashion, e.g., before, during and after a lane changing process.
According to another feature of the invention, the steering-aid adjustment signal of the steering control device is divided into discrete intervals, so that the steering angle adjustment device can be constructionally simple and robust, particularly with respect to the connection between a control device provided there and the associated actuation means, or the actuation means themselves.
Very effective control intervention can be achieved by performing the steering adjustment requested by the control at the front axle of the vehicle in addition to the steering adjustment requested by the driver. In addition, it is also advantageous to provide for steering torque compensation to counteract the change in the restoring force exerted on the steering wheel by the tires due to operation of the controller, which is otherwise conveyed to the driver in an unpleasant manner.
According to yet another embodiment of the invention, steering adjustment, requested by the control, at the rear axle of the vehicle is also possible. Particularly when the vehicle is already equipped with electronic four-wheel steering, this arrangement simplifies the device in that it is then sufficient on the actuation element side to supplement the existing electronic control device with an additional input variable which specifies to the four-wheel steering system an additional steering angle to be implemented.